This invention relates to the field of communications. In particular, this invention is drawn to subscriber loop interface circuitry.
A subscriber loop interface circuit typically provides a communications interface between the plain old telephone system (POTS) network and subscriber equipment such as a telephone. The subscriber equipment is coupled to the POTS network of a central office exchange by an analog subscriber line. The analog subscriber line and subscriber equipment form a subscriber loop. The pair of wires presented to the subscriber equipment by the subscriber line are individually referred to as xe2x80x9ctipxe2x80x9d and xe2x80x9cring.xe2x80x9d
The interface specifications of a subscriber loop interface circuit typically require relatively high voltages and currents for control signaling with respect to the subscriber equipment on the subscriber loop. Voiceband communications are typically low voltage analog signals on the subscriber loop. Subscriber loop performance requirements such as common mode voltage specifications are divided into DC requirements ( less than 300 Hz) and AC requirements (300 Hz-3.4 kHz) for tip and ring.
In one implementation, compliance with AC and DC common mode performance requirements is achieved with a single common mode control loop. One disadvantage of using a single common mode control loop is that longitudinal gains of the tip and ring control must be tightly matched at DC when they are not otherwise required to be. Another disadvantage is that components referencing the longitudinal signal       TIP    +    RING    2
must have a large dynamic range to operate over both DC and AC ranges. Another-disadvantage is that the same longitudinal current drive must provide both AC and DC corrections. The use of a single common mode control loop necessarily implies the use of the same feedback signal for both AC and DC common mode control. Although the use of the longitudinal signal       TIP    +    RING    2
as feedback for AC common mode control may be desirable, the DC common mode requirements are typically specified with respect to the tip voltage. Given that the tip voltage cannot be determined from the longitudinal signal, the DC common mode characteristics cannot be adequately controlled to specification.
In view of limitations of known systems and methods, a bifurcated common mode control for a subscriber loop interface circuit is provided. A subscriber loop interface circuit having bifurcated common mode control loops includes a DC common mode control for controlling tip and ring DC common mode characteristics, and an AC common mode control for controlling tip and ring AC common mode characteristics, wherein the AC and DC common mode controls are independent.
In one embodiment, the DC common mode control includes a tip current source for generating a tip current, idt, and a ring current source for generating a ring current, idr, Each of idt and idr is proportional to a difference between a DC tip voltage and a control voltage.
In one embodiment, the AC common mode control includes a tip current source for generating a tip current, iat, and a ring current source for generating a ring current, iar, Each of iat and iar is a function of the AC tip and ring voltages. The currents generated by each AC common mode current source are controlled by a difference between a first gain term times an AC longitudinal voltage and a second gain term times the AC tip voltage. The second gain term enables cancellation of unwanted contributions from the DC control loop in the audio band.
In one embodiment, a subscriber loop interface circuit apparatus includes a first tip current source for generating a DC tip current component, idt, proportional to a difference between a first tip voltage (TIPDC) and a control voltage (Vcmcontrol) such that idt=gdt(TIPDCxe2x88x92Vcmcontrol). A second tip current source provides an AC tip current component             i      at        =                            g          at1                ⁡                  (                                    TIPAC              +              RINGAC                        2                    )                    -                        g          at2                ·        TIPAC              ,
wherein TIPAC and RINGAC correspond to AC tip and ring voltages, respectively. In one embodiment, the subscriber loop interface circuitry includes a first ring current source to generate a DC ring current component idr=gdr(TIPDCxe2x88x92Vcmcontrol) and a second ring current source to generate an AC ring current component       i    ar    =                    g        ar1            ⁢              (                              TIPAC            +            RINGAC                    2                )              -                  g        ar2            ·              TIPAC        .            
In various embodiment, the gain terms gar1 and gat1 are matched such that gat1≈gar1. Typically, the DC voltages (TIPDC) correspond to tip voltages having frequencies less than 300 Hz. The AC voltages (TIPAC and RINGAC) correspond to tip and ring voltages of frequencies greater than 300 Hz, respectively.
Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below.